The present invention relates to a new and improved method for digital interpolation of a period of a three-phase analog signal, whose components (phases) constitute a periodic function of a measuring magnitude, especially the position during a linear or angular measurement.
Generally speaking, the digital interpolation method of the invention is of the type wherein the values of the components are compared with zero and each two such values compared with one another. Each possible combination of the comparison result has operatively correlated therewith a respective zone which is defined within the range of possible phase argument values of the analog signal, and the value of at least one of the components experiences an analog-digital conversion.
It is already known in the art to produce an analog signal of the previously mentioned type, to digitize the same, and to evaluate it for the purpose of furnishing a measuring value. Significant in this respect is, for instance, Swiss Pat. No. 499,091. It is also known to interpolate the periods of the analog signal, in order to improve the resolution of the digital evaluation. In this respect there is pertinent, by way of example, Swiss Pat. No. 407,569, German Patent Publication No. 2,642,925 and U.S. Pat. No. 3,310,798.
In the case of high precision measuring devices, such as, for instance, flight-path measurement theodolites, it is possible to obtain, by means of the interpolation technique disclosed in U.S. Pat. No. 3,533,097, a suitable resolution. In this United States Patent there is started with a three-phase analog signal, whose phase argument is a linear function of a measured magnitude or quantity, so that the three components (phases) of the analog signal each constitute a respective periodic function of the measured quantity. The values of the components are compared with zero and each two of them with one another. Each possible combination of the comparison result is correlated with a respective zone defined in the range of possible phase argument values of the analog signal. There is thus produced a coarse interpolation of a period of the analog signal into twelve zones of the phase argument. In each zone there is selected the component having the smallest absolute value. The variation of such component with the phase argument is that of a sine function, the argument of which varies between 0 and .pi./12, and thus, the variation of the selected component with respect to the measured quantity deviates from linearity. During the analog-digital conversion of the selected component there is simultaneously compensated the aforementioned deviation, so that there is obtained a digital value, which (considered apart from the quantization) constitutes a linear function of the measured quantity. In this way there is obtained a fine interpolation of the zone into steps, and the number of steps in the zone is equal to the highest possible digital value. The desired result of the interpolation is realized from the combination of the obtained number of zones and the obtained number of steps. Depending upon the obtained number of zones the number of steps should be accounted for positively or negatively, in order to take into account the symmetry properties of the variation of the components with the phase argument.
A first drawback of this interpolation method resides in the fact that, there is required a special circuit design in order to linearize the variation of the digital value with the measured quantity or magnitude. A further drawback is seen in terms of the fact that, the selected component must of necessity be a sine function of the measured quantity, since otherwise the fine interpolation becomes faulty. There is indeed presupposed that each digital value can be unambiguously correlated with a step of the phase argument. In the aforementioned U.S. Pat. No. 3,533,097 there are described measures in order to free such correlation, and consequently, the result of the interpolation, from fluctuations of the amplitude of the sine function. However, to carry out such measures it is necessary to produce the three-phase analog signal as a three-phase alternating-current voltage, and additionally, to provide an alternating-current voltage serving as amplitude and phase reference. Therefore, the teachings of such U.S. Pat. No. 3,533,097 cannot be readily employed for the interpolation of other types of analog signals, particularly not for the interpolation of a signal of the type delivered by the equipment described in Swiss Pat. No. 466,593 for reading Miore-fringe images in a highly precise angle measuring system of a flight-path measuring theodolite. The signal emanating from the aforementioned equipment does not constitute an alternating-current voltage, it is time-dependent only through some possible time-dependence of the measured quantity and, upon standstill of the theodolite, in principle is constant. Additionally, this signal is produced by photocells, so that, on the one hand, the mean or average value of the analog signal might be shifted by a DC-voltage part, and, on the other hand, there can arise at each component appreciable deviations from the ideal sinusoidal course as a function of the measured quantity.